In the manufacture of semiconductor integrated circuits, low resistance interconnection between circuit elements formed on or in the layers of the semiconductor material is essential. Generally, such interconnections are made as a step in the process of microcircuit fabrication utilizing well-known photolithographic masking, etching, and metal deposition techniques. In the past several years, direct writing of interconnects by local energy stimulus of a laser beam has been an active area of research. Such a technique is extremely useful for repairing defects or modifying existing integrated circuits to avoid the necessity for, and complexity involved in, conventional photolithography.
The most widely applied direct writing process has been the pyrolytic deposition of polysilicon from silane. When the silane gas is heavily doped with a material such as diborane, moderately conductive interconnects (500 microohm-cm at best) are deposited. This system has been applied to repair or modification of CMOS gate array circuits (Ehrlich et al. (1982).sup.(1) and other electronic devices.
Many experimental approaches for directly depositing metals have been attempted in this time period. Most such approaches have been very slow due to chemical or optical limitations, or require use of exotic or highly toxic materials. In virtually every case the writing speed and/or the total thickness of the interconnect material is well below technologically useful values. In the case of polysilicon interconnects, the speed and thickness are adequate, but the conductivity is 2 decades below that of metals.
Tungsten and tungsten silicide have received increasing attention as Very Large Scale Integrated (VLSI) interconnection materials, owing to their low contact resistance to silicon, low susceptibility to electromigration, and high conductivity (6 and 70 microohms cm, respectively)..sup.(2)
There have been chemical vapor deposition processes (CVD) for creating layers of tungsten and its silicides for several years, for broad area coatings [for example, see Shaw and Amick (1970)].sup.(4) The growth rates are usually in the range of 100 nm/min (or roughly 6 micrometers/hr). Various attempts to bring these materials into the discretionary direct writing domain have been published. Berg & Mattox (1973).sup.(5) deposited tungsten using an infrared laser and a WF.sub.6 /H.sub.2 gas mix. In this case, the thicknesses were limited to 100 nm, and the laser power and resultant local heating were extremely high. In addition, good metallic conductivity was not obtained consistently unless high temperature furnace annealing followed laser-induced deposition. Deutsch & Rathman (1984).sup.(5) performed a comparison of conventional CVD tungsten and an ultraviolet-laser enhanced deposition of this material. In their case, the laser was used in a broad fixed-spot mode, and again high temperatures were required to obtain low resistivity material. The growth rates/thicknesses were very similar to those stated above.
More recently, there has been activity in applying an interesting reaction wherein WF.sub.6 reduces solid silicon.sup.(6-8), leaving a tungsten cladding over exposed silicon, but has no effect on background materials, such as SiO.sub.2. This is a selective deposition reaction which forms tungsten on silicon by the silicon reduction of WF.sub.6 : EQU 3Si+2WF.sub.6 .fwdarw.3SiF.sub.4 .uparw.+2W
This process has been extensively investigated [Tsao & Busta (1984).sup.(6) ], and many others, for use in improving the conductivity of polysilicon interconnects, which are frequently used in silicon devices. Herman et al. (1984).sup.(3b), and Liu et al. (1985).sup.(3a) have used the reaction to directly write tungsten coatings on bulk silicon, but the thickness is limited to 100 nm, and in most circuit applications, one requires electrical isolation between the silicon substrate and the interconnection metal to prevent shorting of circuit devices.
Therefore, despite the above intensive investigatory efforts, a suitable process for deposition of a low resistivity refractory metal pattern with good adherence on a substrate at high linear writing rates and at sufficiently low temperature to avoid damaging the substrate, has not been reported.